Content based coding and manipulation is one of the core functionalities supported by the emerging MPEG standards. This functionality requires representation and coding of arbitrarily shaped image objects, where the coding problem may be separated into texture and shape (contour) coding.
Multiple layer shape representation is important in the context of object-based image and video coding since it facilitates bit stream scaleable shape coding, where the coded bit streams belonging to different hierarchy layers are separated. If the channel is severely band-limited, only the bit stream belonging to the base layer may be transmitted. Additional layers are transmitted when the channel bandwidth is sufficient. Bit stream scalability is therefore rather important for channels with non-guaranteed bandwidth such as the Internet.
In cases where the entire shape information may be transmitted or stored, bit stream scalability enables the decoder (i) to decode a certain number of layers to meet its computational and display requirements, or (ii) perform a fast decode, using only the base layer, for instance in applications that require low-delay, fast browsing of an image and video database. The decoder may progressively decode and reconstruct the shape information.
Bit stream scalability requires that vertices belonging to different layers are stored separately so that reconstruction of a shape may be performed using only the vertices belonging to a certain selected layer and the previous layers. One of the problems associated with multiple layer, vertex-based shape representation is the problem of preserving the order information of the vertices so that the shape may be reconstructed without ambiguity when all the vertices are not available in their natural order in a single list.